A. Technical Field
The present invention relates generally to root-mean-square (hereinafter, “RMS”) detectors, and more particularly, to the implementation of a true RMS detector comprising multiple variable-gain control stages utilizing automatic variable-gain control relating to changes in an input RF signal.
B. Background of the Invention
Various structures and methods are available for the detection and measurement of voltage and/or power on a radio-frequency (“RF”) signal. RMS detection is an example of a method for detecting and quantifying a power or voltage level over a complete cycle of a sinusoidal signal. In many instances, RMS detection is preferred over peak detection because it is a more accurate measurement of the RF power in an alternating current or voltage signal.
RMS detectors function by squaring an input signal, taking an average of the squared signal over a period of time on the signal, and then square-rooting this average. FIG. 1 illustrates an exemplary prior art RMS detector, which detects an RMS voltage of an incoming signal. The incoming signal 110 is received at an input on a voltage squarer 120 that squares the signal 110. Depending on the design of the detector, the squarer 120 may also scale the signal by a scaling factor (K) and generate an output voltage (V1) 125. It is important to note that the voltage swings on the output voltage (V1) 125 are related to the squared value of the voltage swings on the input signal 110 as shown below.V1=KVi2 
Because of the squaring function of the squarer 120, voltage variations on the input signal 110 are greatly amplified and may result in very large variations in the squarer output voltage (V1) 125. These large voltage variations on the squarer output voltage (V1) 125 may create significant noise offset issues on the low end of the signal and overload problems on the top end of the signal. Accordingly, a squarer may be forced to operate outside of its squaring region, which could effectively clip the output voltage 125 at the high end of the signal as well as cause the squarer to lose its squaring functionality and generate distortion on the squared signal in response to certain input signals and modulation schemes.
The squarer output voltage (V1) 125, and the distortions therein, are provided to an RC circuit 130 that averages this output voltage 125 over a particular cycle of the signal. This averaged or mean value (V2) 135 of the squarer output voltage (V1) 125 is shown below:V2=K Vi2
This mean value (V2) 135 is provided as a first input on a gain block 140 that is coupled across a square-rooter 150. The output (V3) 155 of the square-rooter 150 is provided as a second input on the gain block 140. This gain block 140 effectively forces the square-rooter output (V3) 155 to be equal to the mean value (V2) 135. Accordingly, this output (V3) 155 on the square-rooter 150 is defined as:V3=KVo2=V2=K Vi2
This relationship between the square-rooter output (V3) 155 and the mean value (V2) 135 results in an output of the gain blocker 140 being the RMS value of the input signal 110 and defined as:Vo=√{square root over ( Vi2
As discussed above, the RMS detector in FIG. 1 is limited in the types of signals that it can properly process. This limitation prevents this type of RMS detector from properly functioning in certain types of communication systems that use modulation schemes that generate large peak-to-average ratios on the signal.
Accordingly, what is needed is an RMS detector that is able to function within more diverse types of communication systems including those systems that employ modulation schemes that generate large peak-to-average ratios on a signal.